In the field of motor vehicle drive technology, it is generally known that an electric machine may be used as the drive alone or jointly with a drive motor of a different type (hybrid drive). Electric machines are typically used as the drive motor in such electric or hybrid vehicles. For controlling such electric machines in a motor vehicle, a power electronics device is used, including an inverter which converts the direct voltage/direct current of an onboard (high voltage) battery of the motor vehicle into an alternating current. The inverter generally has a plurality of controllable power switches. The power switches are controlled pulse-width-modulated by a control unit in such a way that the electric machine generates a certain torque at a certain rotational speed of an output shaft of the electric machine during a motor operation.
It is known that a field-oriented regulation (also known as vector regulation) may be used for controlling such inverters for electric machines. In this case, a space vector (for example, a current vector) is moved, rotating with the output shaft of the electric machine. In other words, the phase currents required for controlling the electric machine are converted into a coordinate system (so-called dq system) which rotates with the magnetic field of the machine and is fixedly mounted on the rotor. In fiend-oriented regulation, current components Id and Iq, which are transformed in this way, are then regulated instead of the phase currents.
If a purely electrically driven vehicle or a hybrid vehicle is to be driven by the electric machine, it may happen that a very high startup torque is necessary to move the vehicle from a standstill. This may occur, for example, when starting on a hill, during trailer operation or when driving over a curb or when the weight of the vehicle is relatively high with respect to the maximum power of the electric machine.
When the vehicle is at a standstill, the electric machine is also at a standstill. At a standstill and when starting up from a standstill, the power switches are loaded unevenly since in this state the same power switch(es) will always carry the current required for the startup or standstill torque. Motor startup operation is understood below to refer to operation of the electric machine at a standstill or at a very low rotational speed at which this uneven load on the power switches may occur. If multiple power switches are carrying current in this state, then one of them will generally be under a particularly heavy load, namely the one having the maximum current flow.